Scientists and managers generally believed that biological control is a more cost-efficient and environmentally-friendly way to control pest plants than chemical herbicides and mechanical control. In a recent set of papers, however, some ecologists warn about the potential risks involved with releasing exotic biocontrol agents. They point out that regardless of rigorous precautions taken prior to the release of an exotic species, the potential for adverse effects on non-target organisms and community and ecosystem processes will always exist because biocontrol agents can reproduce, migrate, and evolve. Lafferty and Kuris, in particular, argue that if we consider both ecological and economin principles in making decisions about appropriate biocontrol agents our first question would be "Are native enemies present?". Only after responding negatively to that question should we ask "Do natural enemies exist where the pest is native?" (Lafferty and Kuris, 1996) . Thus, the use of exotic species in biocontrol should only be attempted after biocontrol by native species has been ruled out.

Scientists and agency personnel seem to have taken the opposite approach in the attempt to control purple loosestrife (Lythrum salicaria) in North American wetlands, where it is has become a pervasive pest. Those in charge apparently never asked themselves "Are native enemies present?". Rather, they enlisted the tenets of classical biological control and focused on foreign insects that feed on purple loosestrife in Europe, finally settling on a root-mining weevil Hylobius transversovittatus, a flower-feeding weevil Nanophyes marmoratus, and two leaf-feeding beetles, Galerucella calmariensis and G. pusilla (Malecki et al. 1993). After host-specificity screening, all of these exotic biocontrol agents have been purposefully introduced and established in North America. Thus far there have been no documented problems. Unfortunately, scientists cannot accurately predict if these newly-released exotic species will develop a taste for native plants or how they will influence complex ecosystems.

If the decisionmakers had asked "Are native enemies present?, they would have found at least one species—a leaf-feeding beetle from North America, Galerucella nymphaeae. This beetle is congeneric with the two exotic leaf-feeding beetles recently released in North America, raising the possibility of the exotic species hybridizing with the native beetle. Although Galerucella nymphaeae normally uses water-lilies (Nuphar spp.) and smartweeds (Polygonum spp.) as its host plant, I have observed that this North American beetle is sometimes found on loosestrife in the field and can develop from egg to adult on a diet of purple loosestrife in the lab (Cronin et al. in prep). I have also observed one population of the native beetle feeding exclusively on water shield (Brasenia schreberi), evidence that Galerucella nymphaeae may be able to shift onto new host plants. Thus, it seems prudent to enhance the population sizes of the native beetles before resorting to the more ecologically risky and more expensive exotics.

The potential for using native herbivores to control exotic plants is largely unexplored, but promising. For example, a native weevil (Euhrychiopsis lecontei) has reduced the abundance of the exotic weed Eurasian watermilfoil (Myriophyllum spicatum) in several lakes (Sheldon and Creed 1995). Indeed, the fact that most introduced species do not become established or reach nuisance densities may often result from interactions with native species, including herbivores.

If I may liken exotic species to diseases of native habitats, then classical biological control is analogous to treating a virulent disease with other diseases, the results of which are unpredictable. It seems more desirable to treat the disease by enhancing the native defense mechanisms--the ecological immune system—to control exotic plant pests whenever possible.

References

Cronin, G., T. Schlacher, and D. M. Lodge. in prep. Feeding preference and performance of Galerucella nymphaeae (Chrysomelidae; Coleoptera) on aquatic macrophytes: Are host preferences plastic?

Kareiva, P. (ed.) 1996. Special Feature: Contributions of Ecology to Biological Control. Ecology 77:1963-2013.

Lafferty, K. D. and A. M. Kuris. 1996. Biological control of marine pests. Ecology 77: 1989-2000.

Malecki, R. A., B. Blossey, S. D. Hight, D. Schroeder, L. T. Kok, and J. R. Coulson. 1993. Biological control of purple loosestrife. Bioscience 43: 680-686

Sheldon, S. P. and Creed, R. P. 1995. Use of a native insect as a biological control for an introduced weed. Ecol . Appl. 5: 1122-1132